Generally, pneumatic percussion machines comprise a hollow steel casing or cylinder in which a piston is adapted to be reciprocated. One end of the cylinder is closed off with a valve assembly through which compressed air is supplied to the piston face while the other end is closed off with a bit assembly.
Various arrangements have been proposed for bringing about reciprocation of the piston in the casing. One such arrangement is described in the complete specification to our South African patent application No. 73/7904. In the preferred form of the arrangement described in such complete specification there is provided a projecting member which extends from the valve assembly through the hollow steel casing into a bore provided in the piston. Such projecting member includes longitudinally extending two paths, one path being adapted to introduce compressed air from the valve assembly through the piston to a chamber located at or towards the lower end of the piston. The other path extends from a chamber formed between the top of the piston and the valve assembly, to the bore of the piston, the arrangement being such that for particular positions of the piston relative to the hollow steel casing, air is discharged from this chamber through the projecting member into the bore of the piston from where it can escape to the atmosphere.
In the case of the abovementioned pneumatic percussion machine, there are therefore two paths that pass through the projecting member, one path being intended for introducing compressed air to one of the chambers and the other path being intended for exhausting air from the other chamber to the atmosphere. While no difficulty is encountered, from the engineering point of view, in providing a projecting member having two passages, it is believed that by eliminating one of the passages in the projecting member, the cost of manufacturing such pneumatic percussion machine can be lowered.
An object of the present invention is the provision of a pneumatic percussion machine in which the need for two longitudinally passages in the projecting member is dispensed with.